As is well known, centrifugal compressors pass a fluid such as air or gas in a radial direction of a rotating impeller, and compress the fluid using a centrifugal force generated at that time. Among these centrifugal compressors, a multistage centrifugal compressor that includes impellers in multiple stages in a direction of an axis and gradually compresses a fluid is known.
To be specific, the centrifugal compressor includes a casing, and a rotor housed in the casing. The rotor has a shaft and an impeller fixed to an outer surface of the shaft. A fluid suctioned from a suction port of the casing is given a centrifugal force by the impeller, and kinetic energy thereof is converted into pressure energy by a diffuser and a scroll part. The fluid is sent out of a discharge port of the casing.
According to the requirements of various plants, various centrifugal compressors are produced. In recent years, a centrifugal compressor for compressing a fluid of ultralow temperature (e.g., −160° C.) has been developed, for example, as a compressor for an LNG boil off gas (e.g., see Patent Document 1).